1. Field of the Invention
The present invention relates to a grid interconnection device, a grid interconnection system, and an electric power control system configured to convert electric power from a power supply device into predetermined alternating-current power and interconnects the power supply device to a power distribution system.
2. Description of the Related Art
Heretofore, there has been a proposal of a grid interconnection system for home use, which interconnects a power supply device such as a solar power generator, a wind power generator, an engine generator, a storage battery, or a capacitor to a utility power distribution system.
Here, the grid interconnection system includes a detector that detects an islanding state in the event of a power failure in the power distribution system. The grid interconnection system is configured to perform power system separation from the power distribution system so as to prevent islanding when the detector detects the islanding state. For the detector, moreover, a passive method and an active method are specified as methods for detecting the islanding state, as shown in FIG. 1 (see, for example, “Guidelines on distributed power supply grid interconnection technology, JEAG-9701-2001, pp. 37-47,” Japan Electric Association, Distributed power supply interconnection technical divisions).
When a power supply device shifts to an islanding, the detector using the passive method detects the islanding state by detecting a change in voltage phase, frequency or the like fed from the power distribution system.
In the active method, a variation is constantly given to active power output, reactive power output or the like of the power supply device. The variation is not detectable when there is power supply from the power distribution system. The detector using the active method detects an islanding state by detecting a change in the frequency or the like, which occurs when the power supply device shifts to the islanding.
As mentioned above, the grid interconnection system prevents the islanding in the event of the power failure in the power distribution system and thereby stops a reverse power flow toward the power distribution system, by using the detector using the passive method or the active method.
However, if plural power supply devices are connected in a given range of bank, and for example, if output power from the power supply devices in the bank balances with power consumption of load in the bank, the islanding state cannot be detected in some cases even though the power failure occurs in the power distribution system.
For a solution to this problem, a “transfer trip method” has recently been proposed (see, for example, Japanese Patent Publication No. H8-19183). In the grid interconnection system using the “transfer trip method,” upon occurrence of a power failure in the power distribution system, a transfer trip device of the power distribution system transmits power failure information to the grid interconnection systems in order to inform them of the occurrence of the power failure. The grid interconnection system is configured to stop the operation of the power supply device, upon receipt of the power failure information, according to the power failure information so as to prevent the islanding.